Fresco and wall painting
Some pigments, such as "oropiment, cinnabar, azurite, minium, verdigris, lac and lead" (Mora et al. 1984, 143) cannot be used in fresco because they degrade on contact with lime. The effects of lime on these pigments were known in Roman times: Pliny (NH 35, 49) informs that they could be used on lime and crete mortars, but not in wet applications, and Vitruvius (7,5,8) states that these very pigments were commonly used in wall paintings1. This indicates that techniques including fresh lime and pigments were not used in wall painting, which was executed on dry plaster. The presence of these pigments in Roman wall paintings is very common, according to both Vitruvius and Pliny, suggesting that these paintings were not frescoes. Studies of pigments in Roman paintings corroborate the words of Pliny and Vitruvius about the extensive use in wall paintings of these pigments that degrade on contact with lime. Thus, cinnabar provides the background colour covering entire fields in wall paintings of the Villa dei Misteri in Pompeii or the House of Augustus on the Palatin (Alix Barbet, L´emploi des couleurs dans la peinture murale romaine antique. Pigments et Colorants de l´Antiquité et du Moyen Age. Colloque International du CNRS, 1990, 256-257).